1. Field of the Invention
The present invention relates to an image forming apparatus and a toner concentration sensor.
2. Discussion of the Background
Some of electrophotographic image forming apparatuses use toner. For example, in color printing, a toner image is first-transferred to a toner carrier at an image processing unit. The toner carrier is then brought into contact with a sheet of recording medium conveyed in a predetermined direction to second-transfer the toner image to the sheet. The image transferred to the sheet is then fixed at a fixing unit. The electrophotographic image forming apparatuses use feedback control including measuring toner concentrations prior to the second transfer to reflect the measured toner concentrations to the forming conditions of toner images.
Reflective concentration sensors are used to measure toner concentrations. A reflective concentration sensor irradiates toner with light using a light emitting element of LED or other material. The light that reflects from the toner is received by the reflective concentration sensor at its light receiving element of a photodiode or other material. The reflective concentration sensor converts the received light into electrical energy and calculates a toner concentration based on the intensity of the electrical energy. That is, reflective concentration sensors utilize the phenomenon that light reflectance changes in accordance with toner concentration.
Color image forming apparatuses use four colors of toner, namely, yellow, magenta, cyan, and black. The yellow, magenta, and cyan colors of toner contain many diffuse reflection components. In view of this, diffuse reflection light is used to measure the concentrations of color toner. For the black toner, which completely absorbs light, specular reflection light is used.
Japanese Unexamined Patent Application Publication No. 10-333416 discloses means for sensing the concentration of a toner image of a plurality of colors. Specifically, a plurality of light emitting elements are used to emit different wavelengths of light that are predetermined in accordance with the spectral reflectances of the plurality of colors of toner. In accordance with the color of toner to be sensed, a corresponding one of the light emitting elements is used to emit light. This ensures a highly sensitive measurement of concentration for individual colors.
Japanese Unexamined Patent Application Publication No. 2002-148887 discloses a concentration sensing device that includes two light emitting elements. One of the light emitting elements is designed for specular reflection light to sense the concentration of black toner. The other light emitting element is designed for diffuse reflection (scattering reflection) to sense the concentration of colored toner. It is somewhat time-consuming to stabilize the amount of light radiated from the light emitting elements. In view of this, a certain specific timing is used for light emission of the light emitting element for specular reflection light and the light emitting element for diffuse reflection. This ensures a highly accurate and high speed measurement of the concentration of the toner image.
The contents of Japanese Unexamined Patent Application Publication No. 10-333416 and Japanese Unexamined Patent Application Publication No. 2002-148887 are herein incorporated by reference in their entirety.
Incidentally, when reflective devices are used to sense toner concentration, their sensitivity increases as the incident angle of the incident light and the reflection angle of reflecting light reduce (the incident light is radiated from the light emitting element and incident on the toner carrier, and the reflecting light is reflecting from the toner image toward the light receiving element).
Further, some intermediate transfer belts serving as toner carriers have transparent films deposited over elastic substrates. In this case, incident light is first refracted on the surface of the transparent film and then reflected on the surface of the substrate. This indicates that the spectral reflectance of an intermediate transfer belt varies depending on the thickness of the transparent film and on the incident angle.
The conventional techniques, such as those disclosed in Japanese Unexamined Patent Application Publication Nos. 10-333416 and 2002-148887, give no consideration to the fact that the spectral reflectance varies depending on the thickness of the transparent film of the intermediate transfer belt and on other parameters. Accordingly, the light for specular reflection and the light for diffuse reflection are set at the same spectral reflectance. More specifically, the light for specular reflection and the light for diffuse reflection on toner both end up with specular reflection on the intermediate transfer belt.
However, if the light for diffuse reflection on toner undergoes specular reflection on the intermediate transfer belt and then enters the light receiving element, the sensing of toner concentration is inaccurate. In view of this, a conventional light emitting element to sense toner concentration by diffuse reflection has an optical axis that is spaced apart from the optical axis of the light receiving element, in an attempt to prevent specular reflection light from the intermediate transfer belt from entering the light receiving element. This necessitates the light emitting element and the light receiving element to be spaced apart from one another. This in turn increases the incident angle and the reflection angle of light on the intermediate transfer belt for one light emitting element, causing a problem of degraded sensitivity (accuracy) to toner concentration.
Additionally, another light emitting element among the plurality of light emitting elements needs to be set to emit light to undergo specular reflection on the intermediate transfer belt. This necessitates the plurality of light emitting elements to be spaced apart from each other, causing a problem of an enlarged concentration sensing device.